Among technologies widely used for cooling of chilled water for air cooling and the like, there are a falling film heat exchanger and an absorption refrigeration system using this heat exchanger. This falling film heat exchanger is a heat exchanger provided with a tube bank formed of numerous heat exchanger tubes, through each of which a first fluid (liquid or gas) flows, arranged not only in a horizontal direction and but also in a vertical direction. A second fluid (liquid) is dropped or spayed, for example, to flow down onto the outer surfaces of the heat exchanger tubes in the uppermost array, so that the outer surfaces of the heat exchanger tubes are covered with the second fluid. Thereby, the first fluid and the second fluid are caused to exchange heat. Then, the second fluid covering the outer surfaces of the heat exchanger tubes in that array is caused to flow down to the heat exchanger tubes in the next array below these heat exchanger tubes. Thus, through the heat exchanger tubes in the next array, the first fluid and the second fluid exchange heat. In this way, the heat exchanger causes the first fluid and the second fluid to exchange heat through the heat exchanger tubes in the lower arrays sequentially one after another.
This falling film heat exchanger can perform efficient heat exchange because the heat exchanger can achieve a great leap in the heat exchange amount by use of the heat of vaporization of the second fluid. For this reason, such falling film heat exchangers are widely used for evaporators, absorbers, regenerators and the like of absorption refrigeration systems.
Meanwhile, an absorption refrigeration system described in International Patent Application Publication No. WO98/41798, for example, uses the second fluid as a coolant in an evaporator, and is configured to promote evaporation of the coolant in a way that the coolant is caused to flow down onto the outer surfaces of heat exchanger tubes in a closed vessel, and to exchange heat with the first fluid in the heat exchanger tubes in the process of flowing down, thereby generating a gas of the coolant; and the gas is absorbed by an absorbing liquid in an absorber communicating with the closed vessel. For the promotion of the evaporation of the coolant, it is important to distribute the coolant forming a falling film evenly over the entire length of each of the heat exchanger tubes, and to surely distribute the coolant over the bank of heat exchanger tubes.
To this end, it is necessary to devise a method of distributing and dropping the coolant or the absorber onto the heat exchanger tubes in the uppermost array as evenly as possible, and to also space out the heat exchanger tubes at certain intervals. In addition, in the case where the heat exchanger tubes arranged to extend horizontally are used, the configuration of a liquid distributor is especially important because the liquid dropped down from the heat exchange tubes in an upper position need to be surely dropped onto the heat exchange tubes in a lower position. As such a liquid distributor, a dropping device made of a single plate and being usable as a dropping device for an absorption refrigerator has been proposed as described in Japanese Patent Application Publication No. 2005-207620, for example.
Moreover, as described in Japanese Patent Application Publication No. Hei 11-108501, for example, an evaporator for an absorption refrigerator has been proposed in order to improve heat exchange performance by preventing an increase in a dried portion where no coolant liquid flows down on the surfaces of heat exchanger tubes. The evaporator is provided with spacers arranged at predetermined intervals in a vertical direction, located between upper and lower neighboring heat exchanger tubes and extended in a longitudinal direction of the heat exchanger tubes, and is configured to form liquid pools around joint portions between the spacers and the lower heat exchanger tubes.
However, this falling film heat exchanger and an absorption refrigeration system using this heat exchanger have drawback in that the exchanger can hardly tolerate an inclination with respect to the horizontal plane and a swing motion because the performance is remarkably deteriorated unless the coolant or absorber is dropped while being distributed evenly over the heat exchanger tubes in the uppermost array, or unless the liquid flowing down from each heat exchanger tube in the upper position surely flows down onto the surface of the heat exchanger tube in the lower position and covers the entire surface with its liquid film. This drawback becomes more serious if the falling film heat exchanger inclines. Hence, there arises a problem that this falling film heat exchanger and the absorption refrigeration system using this heat exchanger are not installable in a ship, an offshore structure, or an underwater offshore structure which inherently involves an inclination and a swing motion.
In short, in connection with the inclination of the falling film heat exchanger, there are a bypass problem, a no-flow region increase problem and a wet surface reduction problem. The bypass problem is a problem that the second fluid passes through between the heat exchanger tubes in the first next lower array, and flows down to the heat exchanger tubes in the second next lower array.
As illustrated in FIG. 13, in the case of a falling film heat exchanger employing a configuration in which heat exchanger tubes 21 are aligned in a staggered arrangement, a second fluid D is dropped toward the center lines (tube axes) of the heat exchanger tubes 21 in the uppermost array when the falling film heat exchanger is not inclined in a cross-sectional plane of the heat exchanger tubes 21. In this case, however, the dropping second fluid D may bypass the heat exchanger tubes 21 in the even-numbered arrays from the top by passing without contacting the heat exchanger tubes 21.
In the case of water, it is said that a volume of 20 droplets is about 1 cc and a natural droplet diameter of the second fluid D is only about 2 mm. For this reason, this problem cannot be avoided unless the heat exchanger tubes 21 are arranged at lateral intervals S of 2 mm or smaller. Nevertheless, it is difficult to make the lateral intervals be 2 mm or smaller in view of process working of a tube sheet (tube wall) and strength.
Instead, as illustrated in FIG. 14, in a case of employing a non-staggered arrangement, this bypass problem does not occur unless an inclination at about 20 degrees occurs. However, as illustrated in FIG. 15, once the inclination occurs at an inclination angle exceeding about 20 degrees, the bypass problem also occurs as in the case of the staggered arrangement, and the heat exchange area is reduced by as much as 50% only due to the bypassing.
Next, the no-flow region increase problem is a problem that a region of the heat exchanger tubes 21 where the second fluid D does not flow, in short, a no-flow region occurs due to a sideways inclination of the apparatus. If the exchanger inclines largely, there are some heat exchanger tubes 21 (hatched heat exchanger tubes) onto which the second fluid D does not flow down, as illustrated in FIG. 16. In the case of employing the non-staggered arrangement as illustrated in FIG. 16, this phenomenon does not occur unless the inclination of the apparatus reaches an inclination at about 20 degrees. However, once this phenomenon occurs, the heat exchange area is reduced as illustrated in FIG. 16, and about 50% of the total region is a no-flow region in the case of FIG. 16. On the other hand, in the case of employing the staggered arrangement as illustrated in FIG. 17, even a slight inclination influences about 20% of the heat exchange area in the case of FIG. 17. Nevertheless, even if the inclination angle increases, the influence more than that does not occur unless the inclination angle reaches about 40 degrees. Although FIGS. 16 and 17 illustrate the cases where the heat exchanger tube bank includes the same number of heat exchanger tubes arranged in the vertical direction and in the lateral direction, the no-flow region increase problem occurs more notably if the number of tubes arranged in the vertical direction is larger than that in the lateral direction.
Then, the wet surface reduction problem is a problem that the liquid film does not cover the entire surface of a heat exchanger tube 21 but covers only a part of the surface. If each heat exchanger tube 21 receives a droplet at a position thereof in the plumb line (from right above), the liquid film Ds is formed on both the right and left sides of the heat exchanger tube 21 as illustrated in FIG. 18. However, if the droplet is slightly displaced, the liquid film Ds is formed on only one side as illustrated in FIG. 19. In the case illustrated in FIG. 19, the liquid film Ds is reduced to about 35% whereas about 65% of the total is not covered with the liquid film Ds. If the droplet grazes the heat exchanger tube 12 very slightly, in particular, the liquid film Ds is reduced to about 25% of the total whereas about 75% of the total is not covered with the liquid film Ds. In the case of the staggered arrangement, this problem especially occurs even if the inclination is not large.
Hence, if the inclination of the apparatus is about 20 degrees or smaller, and if the interval between the heat exchanger tubes 21 has a certain distance, it can be said that the non-staggered arrangement can easily avoid the bypass problem and the no-flow region increase problem. In the case of the non-staggered arrangement, however, it is considered that, due to the wet surface reduction problem, the apparatus can exert only 50% or less of its full performance with a somewhat large inclination large, or only about 35% thereof with an inclination at about 20 degrees.
On the other hand, in the case of the staggered arrangement, it is important to arrange the heat exchanger tubes at horizontal intervals of about 2 mm or less in order to prevent a phenomenon in which the bypass problem occurs in the even-numbered arrays even when the inclination is not large. Even if such spacing is realized, it is considered that, due to a reduction in the heat exchange area due to the no-flow region increase problem and a reduction in the heat exchange area due to the wet surface reduction problem, the apparatus can exert only about 30% of the full performance if the heat exchanger tube bank includes the same number of heat exchanger tubes arranged in the vertical direction and in the lateral direction, or exert much poorer performance if the number of tubes arranged in the vertical direction is larger than that in the lateral direction.